Third Law

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Third Law of Thermodynamics states entropy of a perfect crystal at 0 K is:

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1. **Entropy and Temperature**: Entropy measures molecular disorder. At lower temperatures, thermal motion decreases, and disorder drops.
2. **Third Law Postulate**: The entropy of a **pure, perfectly crystalline** substance is **exactly zero** at the absolute zero of temperature ($0$ Kelvin).
3. **Why?**: In a perfect crystal, there is only one possible microstate: every atom is in its exact specified lattice position. According to Boltzmann's formula $S = k \ln W$, when $W=1$, $S=0$.
4. **Reference State**: This law allows scientists to define a 'Standard Entropy' (an absolute scale) for all substances.

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Third Law

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